How to set vertex colors on import
I think it will work if you do it like...
mesh.vertex_colors.new(name='layer0',do_init=False)
mesh.vertex_colors.new(name='layer1',do_init=False)
mesh.vertex_colors.new(name='layer2',do_init=False)
mesh.vertex_colors.new(name='layer3',do_init=False)

vertex_color_layer0 = mesh.vertex_colors['layer0']
vertex_color_layer1 = mesh.vertex_colors['layer1']
vertex_color_layer2 = mesh.vertex_colors['layer2']
vertex_color_layer3 = mesh.vertex_colors['layer3']

for i, c in enumerate(data.colors):
    vertex_color_layer0.data[i].color = (c[3],c[3],c[3],c[3])
    vertex_color_layer1.data[i].color = (c[0],c[0],c[0],c[0])
    vertex_color_layer2.data[i].color = (c[1],c[1],c[1],c[1])
    vertex_color_layer3.data[i].color = (c[2],c[2],c[2],c[2])